1. Field of the Invention
This invention relates generally to photonic integrated circuits (PICs) and more particularly to optical transmitter photonic integrated circuit (TxPIC) chips having on-chip arrays of modulated light sources, not requiring additional on-chip amplification components.
2. Description of the Related Art
As used throughout this description and the drawings, the following short terms have the following meanings unless otherwise stated:
AWG—Arrayed Waveguide Grating.
BCB—benzocyclobutene or derivatives thereof.
DBR—Distributed Bragg Reflection Laser.
DEMUX—A Demultiplexer.
DFB—Distributed feedback Laser.
EA or EAM—Electro Absorption or Electro Absorption Modulator.
EML—Electro-optic Modulator/Laser.
ER—Extinction Ratio.
GC-SOA—Gain Clamped Semiconductor Optical Amplifier.
ITU Grid—Wavelengths and frequencies used in WDM systems that have been standardized on a frequency grid by the International Telecommunications Union (ITU).
MMI—Multimode Interference.
MOCVD—Metalorganic Chemical Vapor Deposition or organometallic vapor phase epitaxy.
MOD—Modulator.
MUX—A Multiplexer.
NA—Numerical Aperture.
NID—Not Intentionally Doped.
PD—Photodiode
PIC—Photonic Integrated Circuit.
Q—A Group III-V quaternary comprising InGaAsP or InAlGaAs.
QW—Quantum Well.
RxPIC—Receiver Photonic Integrated Circuit.
SAG—Selective Area Growth usually performed in MOCVD.
SOA—Semiconductor Optical Amplifier.
SSC—Spot Size Converter—sometimes called also a mode adaptor.
TxPIC—Transmitter Photonic Integrated Circuit.
Wavelength Grid—Wavelengths and frequencies in a periodic or aperiodic frequency grid whether a standardized grid or not.
There exists a great demand at this time that future generations of optical transmitters and optical receivers or optical transceivers for optical telecommunications to be much more cost effective than present optical telecommunication equipment that comprise optical discrete optical components that are separately manufactured, assembled and packaged. It is clear that a solid approach to achieve this goal is a photonic integrated circuit (PIC) that includes, in monolithic form, the integrated arrays of active electro-optic components and optical passive components, i.e., multiple signal channels within a standardized grid where each channel includes a modulated source (which may comprise either a directly modulated laser or a laser and an external modulator, sometimes referred to as a semiconductor modulator/laser (SML), e.g., an EML) coupled to an optical combiner. It has been suggested that transmitter photonic integrated circuits (TxPICs) comprise, in monolithic form, a laser (which may be tunable), and electro absorption modulator (EAM), such as shown in the articles of Thomas L. Koch et al. entitled, “Semiconductor Photonic Integrated Circuits”, IEEE Journal of Quantum Electronics, Vol. 27(3), pp. 641–653, March, 1999 and D. A. Ackerman et al. entitled, “A Practical DBR Laser based Wavelength Selectable DWDM Source”, IEEE LEOS Newsletter, pp. 7–9, October, 2001; DFB laser arrays and EA modulator arrays such as shown in U.S. Pat. Nos. 5,891,748 and 5,784,183; DBR laser arrays, EA modulators, optical combiner and output amplifier on a single chip such as shown in the article of M. G. Young et al. entitled, “A 16X1 Wavelength Division Multiplexer with Integrated Distributed Bragg reflector Lasers and Electroabsorption Modulators”, IEEE Photonics Technology Letters, Vol. 5(8), pp. 908–910, August, 1993. Also, there is the article of Charles H. Joyner et al., entitled, “Low-Threshold Nine-Channel Waveguide Grating Router-Based Continuous Wave Transmitter”, Journal of Lightwave Technology, Vol. 17(4), pp. 647–651, April, 1999 disclosing a single monolithic optical chip, i.e., a photonic integrated circuit (PIC), having a plurality of semiconductor optical amplifiers (SOAs) with their optical outputs coupled via a plurality of passive waveguides to an AWG to form a multiple wavelength laser source having multiple established laser cavities between these coupled optical components. To be noted is that there is an absence in the art, at least to the present knowledge of the inventors herein, of an integrated laser source array, such as in the form of a DFB array, and an optical combiner in the form of an array waveguide grating (AWG). A principal reason is that it is difficult to fabricate, on a repeated basis, an array of DFB lasers with a wavelength grid that matches the wavelength grid of the AWG. Also, as the numbers of electro-optic components are added to a PIC chip, insertion losses increase requiring that some on-chip or off-chip optical signal amplification is included.
It has been suggested that receiver photonic integrated circuits (RxPICs) comprise, in monolithic form, ridge waveguide, arrayed waveguide gratings (AWGs) and an array of photodetectors as shown in the articles of Masaki Kohtoku et al. entitled, “Polarization Independent Semiconductor Arrayed Waveguide Gratings Using a Deep-Ridge Waveguide Structure”, IEICE Trans. Electron., Vol. E81-C, No. 8, pp 1195–1204, August, 1998 and “Packaged Polarization-Insensitive WDM Monitor with Low Loss (7.3 dB) and Wide Tuning Range (4.5), IEEE Photonics Technology Letters, Vol. 16(11), pp. 1614–1616, November, 1998. Another example is the article of M. Zirngibl et al. entitled, “WDM receiver by Monolithic Integration of an Optical Preamplifier, Waveguide Grating router and Photodiode Array”, ELECTRONIC LETTERS, Vol. 31(7), pp. 581–582, Mar. 30, 1995, discloses a 1 cm by 4 mm PIC chip, fabricated in InP, that includes the integrated components comprising an optical amplifier (SOA) optically coupled to an AWG DEMUX having a plurality of different signal channel outputs each coupled to a respective photodiode (PD) in an array of on-chip photodiodes. The SOA boosts the multiplexed input channel signals. The AWG DEMUX demultiplexes the signals into separate channel signals which signals are respectively detected by a PD in the array.
As indicated above, many of the above mentioned PIC devices include an on-chip optical amplifier to boost the power of optical channel signals generated by or received in the PIC, such as a SOA or an optical laser amplifier. These added gain components are useful to enhance the power of the channel signals especially where on-chip insertion loss exceeds the insertion loss budget allowed in the design of such PIC chips. However, the presence of additional active optical components, while solving gain needs, provides additional constraints on the resulting PIC chip thermal budget through the requirement of additional PIC operating power which translates into higher PIC heat generation and required dissipation. Also, the addition of a plurality of SOAs on the TxPIC chip tightens what we term the selective area growth (SAG) budget where the wavelengths of the active/waveguide core of the DFBs, EA modulators and added SOAs, for example, must be monotonically shifted via SAG processing. This results in the bandgap in each consecutive optical component in an optical waveguide formed in the PIC to be optimized for performance. For example, the wavelength of the AWG waveguide region is less than the wavelength of the MOD active region which is less than the wavelength of the DFB active region which is less than the wavelength of the SOA active region (λAWGi<λMODi<λDFBi<λSOAi where λDFBi+1=λDFBi+Δλ, λMODi+1=λMODi+Δλ, λSOAi+1=λSOAi+Δλ and λAWG<<λMODi,). Δλ is the channel spacing. Note that it is  possible to vary the wavelength spacing Δλ1 across the array in a proprietary PIC system.
Also, the presence of SOAs on a monolithic PIC chip increases fabrication and test complexity. Their deployment on the TxPIC side (versus the RxPIC side) can add to unwanted dispersive effects on the transmitted waveform or may otherwise degrade the signal transmission properties. An SOA may amplify the optical reflections between integrated components, resulting in increased and undesireableundesirable back reflection. Further, the addition of on-chip SOAs increases the stress on the available SAG budget, albeit it may be only a same percentage of the total budget, such as around 10%. The SAG budget may be defined as the range of attainable operating wavelengths with sufficient wavelength separation to enable the proper wavelength targets for totally all optical components or devices on the chip. It would be preferred to reserve the SAG budget for DFB laser wavelength budget or for the DFB/MOD wavelength budget by reducing the number of optical components on the chip, in particular, eliminating any need for on-chip SOAs making it easier to optimize the DFB array performance/yield or the DFB/MOD performance/yield or DFB/MOD/MUX performance/yield. Further, the elimination of SOAs from the PIC chip renders it also possible to increase the density of DFBs included on a single semiconductor chip, which translates into an increase in the number of signal channels per TxPIC chip, reducing the cost per channel for a PIC transmitter module.